1. Field of the Invention
This invention relates to a process for producing expandable thermoplastic resin beads, especially those which have superior foamability and the resulting foamed articles made from such beads provide good thermal stability.
2. Description of the Prior Art
Generally, it is easy to obtain polystyrene beads having a high expansion ratio. The resulting foamed articles made from such beads have high rigidity and good shape retention, but have the disadvantage in that they are fragile and have poor chemical resistance, oil resistance and thermal stability. Foamed products of polypropylene resin, on the other hand, have better elasticity, chemical resistance, oil resistance and thermal stability than polystyrene foams. However, polypropylene foams are too pliable, and are unsuitable for the preparation of shaped articles such as containers which retain a certain definite shape.
When polypropylene and polystyrene are simply mixed in an attempt to obtain a resin having the desirable characteristics of both polymers, a uniform mixture cannot be obtained. A foamed product prepared from such a mixture undergoes phase separation, and does not have an attractive appearance. Many investigations have been undertaken to remedy this disadvantage. For example, Japanese Patent Publication No. 26097/72 discloses a process for producing foamable pellets of a vinyl aromatic polymer which comprises heat-melting a vinyl aromatic polymer (polystyrene) containing 1 to 15% by weight, based on the total amount of the resin, of a polyolefin, pelletizing the melt, and impregnating the pellets with a blowing agent. According to this method, the amount of the polyolefin mixed is small, and since the vinyl aromatic polymer is simply physically mixed with the polyolefin, the two resins are not uniformly mixed and phase separation occurs. As a result, the impregnation of the blowing agent and the expansion ratio become non-uniform, and a uniform foamed article cannot be obtained. Moreover, because of the low polyolefin content, the pliability and elasticity of the resulting foamed product cannot be improved.
Japanese Patent Publication No. 32623/70 discloses a process for producing a foamable granular thermoplastic polymer which comprises dispersing in an aqueous medium a granular thermoplastic resin composed mainly of an aliphatic olefin resin, a polymerizable vinyl monomer capable of dissolving or swelling the thermoplastic resin, a polymerization catalyst and a cross-linking agent composed of an organic peroxide, introducing a blowing agent which is normally gaseous or liquid into the aqueous dispersion, and heating the mixture to a temperature at which the vinyl monomer has a solubilizing power for the aliphatic olefin resin and the heat also causes decomposition of the cross-linking agent, whereby the polymerization of the vinyl monomer, the impregnation of the blowing agent, and the cross-linking reaction occur simultaneously under pressure. Since the polymerization of the vinyl monomer, the impregnation of the blowing agent and the cross-linking reaction are performed simultaneously under pressure in this process, a special manufacturing apparatus is required, and the process has disadvantages for commercial mass production. In specific embodiments of this process, the content of polyolefin is very high, and, therefore, the resulting foamable pellets do not have a sufficient gas retaining property. Moreover, the resulting foamed product has the same properties as a foamed article of polyolefin alone.